Look at the docs for glFrustum() and/or glOrtho. That tells you the corners of that frustum (call it frustum B) in that frustum's eye space. You can draw the sided or edges using that. See these good reference pages by Song Ho Ahn:

Before you draw it though, you need to have that space active. If the frustum your using for your currently rendered view is frustum A, put frustum A's VIEWING transform on the MODELVIEW stack. Then add on the "inverse" VIEWING transform for frustum B. Then draw your frustum B sides/edges in frustum B's EYE-SPACE.

Look at the docs for glFrustum() and/or glOrtho. That tells you the corners of that frustum (call it frustum B) in that frustum's eye space. You can draw the sided or edges using that. See these good reference pages by Song Ho Ahn:

Before you draw it though, you need to have that space active. If the frustum your using for your currently rendered view is frustum A, put frustum A's VIEWING transform on the MODELVIEW stack. Then add on the "inverse" VIEWING transform for frustum B. Then draw your frustum B sides/edges in frustum B's EYE-SPACE.

Thank you for quick answer, I will look through it, and if you don't mind, I will ask some questions personally, if I have them.